This invention relates to a polyester film fabricated so as to contain numerous minute cells in the surface and inner parts thereof. More particularly, this invention relates to a stretched polyester film which has a lowered apparent specific gravity owing to formation therein of minute closed cells and is excellent in whiteness, opacifying properties, and adhesive properties.
Heretofore, polyester films have found extensive utility in various industrial fields owing to their excellence in mechanical properties, electrical properties, resistance to chemicals, and resistance to heat. Particularly the biaxially stretched polyethylene terephthalate film excels other films in planarity and dimensional stability and is marketed at a relatively low price and, therefore, it constitutes itself an indispensable materials for printing plates and for substrates in data devices.
The electronic white board is counted among the commodities which are used in offices and meeting rooms. A white biaxially stretched polyester film is used in the copy board part, i.e. an important part, of the electronic white board. White polyester films have been finding utility as substrates for magnetic cards represented by telephone cards and railway cashless cards which are widely spreaded in recent years. In these magnetic cards, the white polyester films are fully manifesting their outstanding properties mentioned above.
Indeed these white films possess highly desirable properties. They nevertheless suffer from various drawbacks because they require to contain a white pigment in a very large amount for the purpose of acquiring an enhanced opacifying properties. Since these films contain inorganic particles such as titanium oxide particles having a large specific gravity at times in a ratio exceeding 10% by weight, they have an apparent specific gravity exceeding 1.7, occasionally exceeding 2.0, and a weight per unit volume becomes 20 to 50% larger than that of their regular countertypes containing no pigment.
This increase in the apparent specific gravity causes accelerated sagging of film with aging under own weight. In the case of the white board mentioned above, for example, the phenomenon of sagging of the board surface with aging is greatly accelerated during long-term use, thereby seriously impairing the commercial value of the white board.
Moreover, the inorganic particles in the white film contribute to increase the rigidity of film and cause shortening the service life of a knife to be used in slitting a film or cutting a card. The film edges formed by slitting or cutting the film of increased rigidity are liable to inflict lacerations on operative's hands. Thus, the white films containing such inorganic particles have difficulties in terms of productivity and handling properties.
Further, as an inevitable consequence, such white films cause heavy labor during the work of packing or transportation.
The inventors of the present invention made a study in search of a way of decreasing the apparent specific gravity and consequently solving the various problems mentioned above. As the result of the study, it was found that the phenomenon of sagging of polyester film with aging under own weight could be notably eliminated by foaming the film and converting it into a minute-cellular film as proposed in Japanese Patent Application No. 61-313896 (1986). The finely dispersed closed cells in the film play a part in imparting opacifying properties to the film and consequently decreasing the amount of pigment to be added to the film and, at the same time, moderating the rigidity of film, lengthening the service life of the knife used for the slitting work, and appreciably eliminating the possibility of sharp film edges inflicting lacerations on operative's hands.
Though the minute-cellular polyester film has attained such outstanding effects as described above, it is not necessarily satisfactory in terms of the other film properties. It has been found that this film exhibits a poor adhesive property to a printing ink etc. as compared with the conventional white film contrary to the expectation that the film exhibits an improved adhesive property because of large surface irregularities thereof. It has been further pointed out that images printed thereon tend to blur. A need has developed, therefore, to provide the film of a quality capable of solving these difficulties.
The practice of subjecting polyester films to various surface treatments to provide the films of an improved adhesive property has been already known widely in the art. The flame treatment, solvent treatment, surface coating treatment, corona discharge treatment, plasma treatment, ultraviolet light treatment, ion plating treatment, radiation treatment, and sand blasting treatment are examples. With respect to these methods of treatment, proposals have been made for the improvement of the adhesive property.
These conventional surface treatments have been employed mainly for usual falt polyester films. They have never been applied to polyester films of the type having such an extremely high surface roughness as in the film according to the present invention.
This is because there has never arisen any need for improving the adhesive property in the case of films having relatively high surface roughness. Specifically, it has been widely understood that the adhesive force exhibited by the film of this nature increases with increasing the surface roughness because the contact are of this film with a given layer increases with increasing the surface roughness and further because an adhesive layer deeply entered into depressed portions of the surface of the film enhances anchoring effect. In fact, these films have yielded results which are accounted for substantially completely by this concept.
In the case of such a film as in the present invention, which has an extremely high surface roughness, the results to be obtained therewith deny the aforementioned conventional concept of adhesive property as described above. This fact clearly indicates that the various conventional method of surface treatment intended for improving the adhesive property of film are not always simply applicable. It is not necessarily easy to expect from the prior art a method for improving the adhesive property of a film having a special surface shape produced by inner cells as in the film of the present invention. Therefore, it may be said what method is effective in improving the adhesive property of the particular film under discussion is quite unknown.
In the circumstances, the desirability of developing a convenient and inexpensive method for improving the adhesive property of cellular films possessing special properties without impairing the inherent properties thereof has found approval.
The present inventors have continued a study with a view to the development of such a method as described above. They have consequently found that a uniaxially or biaxially stretched minute-cellular polyester film having a high level of surface roughness can shown an improved adhesive property by forming a coating on either or both of the surfaces of the film. The present invention has been accomplished on the basis of this finding.